1. Field of the Invention
The present invention relates to fermented milk exhibiting a controlled increase in the acidity during storage and transportation and containing lactobacilli and lactococci at a specific ideal ratio, and to a process for manufacturing such fermented milk.
2. Description of the Background Art
Antibacterial substances produced by lactic acid bacteria, typified by nisin produced by Lactococcus lactis subsp. lactis, formerly called Streptococcus lactis subsp. lactis, are used for the purposes of promoting a storage performance of foods and preventing quality of foods from being deteriorated due to proliferation of bacteria in the foods and contamination thereof.
It is very difficult to retain through the entire period of storage and transportation favorable flavor possessed by fermented foods immediately after the production, e.g., fresh flavor of fermented milk, in which living bacteria are present. Yogurt, for example, which is characterized by its containment of living bacteria, possesses an inherent defect of its flavor change due to the increase in acidity during storage and transportation. A number of trials have been undertaken for preventing the acidity of yogurt from being increased during storage. All of them, however, brought about only insufficient results; some exhibited only poor effects of preventing the acidity increase, others required very complicated manufacturing processes, and many of them involved high production costs.
As mentioned above, living bacteria contained in fermented food products, such as yogurt, proliferate during the storage and increase the acidity of the products, leading to deterioration of the flavor.
Development of a process for preventing such deterioration of flavor in fermented food products by suppressing activities of bacteria, which is a cause of such flavor deterioration, by a simple method without altering conventional process steps has been desired.
Among major bacteria used for fermentation of raw milk at the start of yogurt production, Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus salivarius subsp. thermophilus, the former is the cause of the acidity increase at least in the storage and transportation period. Controlling the growth of Lactobacillus delbrueckii subsp. bulgaricus to directly inhibit the acidity formation of the bacteria during the storage, therefore, contributes to the prevention of the acidity increase in the product, leading to efficient prevention of the quality deterioration.
From this aspect, the present inventors have undertaken extensive studies on the mutual actions between bacteria used at the start of fermentation (such bacteria are hereinafter called "starter bacteria" or simply "starter") in manufacturing processes of fermented foods, and found that the growth of starter bacteria in the raw milk is suppressed by certain bacteria of Lactococcus lactis subsp. lactis. The inventors have further studied the cause of such inhibition and found that an antibacterial agent nisin secreted by Lactococcus lactis subsp. lactis in a medium suppresses the growth of other starter bacteria and controls formation of acids. These findings have led to the completion of the present invention.